Steroid hormones produce some remarkable effects in hamsters. The hepatic synthesis of a pentraxin protein, female protein (FP), is controlled by sex steroids in the Syrian hamster. The resulting high serum levels in female hamsters is directly associated with another unique phenomenon, i.e., a sex limited expression of amyloidosis which predominantly affects female hamsters. FP is a constituent of hamster amyloid and the presence of amyloidosis (incidence and amount) was diminished by administration of exogenous steroids which depressed hepatic synthesis of FP. The role of FP in hamster amyloidosis is thought to be analogous to that of its homolog, serum amyloid P component (SAP), in human amyloidosis; that is, SAP incorporation is depicted as a secondary event after formation of fibrils, the main constituent of amyloid. These observations in hamsters indicate that the P component homolog (FP) has a primary role in hamster amyloidosis. In contrast to Syrian hamsters, estrogen (DES) administration induces an opposite pentraxin response in Armenian hamsters, as FP synthesis is inhibited. Also, estrogens induce an acute toxicity (icterus) in Armenian and Chinese hamsters (but not Syrian hamsters), and these effects are mediated by estrogen receptors. Finally, chronic estrogen infections can induce appearance of multiple tumors in the livers of both male and female hamsters.